The present invention relates to tire building drums, and more particularly to a ply turn-up device for turning carcass plies around the bead wire.
In one well-known type of these devices, an inflatable bladder having the general shape of a torus is used. In order to assure the transverse displacement of these bladders, which displacement is necessary in order to effect the turning up, it is necessary to have recourse to an auxiliary element which effects a transverse push on the ply turn-up bladder. This auxiliary element is either a second inflatable bladder or else a metal ring which comes into contact with the turn-up bladder. One example of such a device is given in U.S. Pat. No. 3,698,987.
A ply turn-up bladder which by itself, solely under the effect of the increase in the inflation pressure, develops the rolling movement in the transverse direction which is necessary in order to effect the turning up of the carcass ply is also known. This ply turn-up bladder is described in U.S. Pat. No. 4,087,306.
The drawback of all of these known devices results from the size of their diameter, which is generally very great, to the extent of coming into collision with the passages for the placing of the carcass plies and other products brought onto the drum of the tire. This large diameter is particularly disturbing when the drum is moved from station to station in a tire manufacturing line.
Devices in which the rolling movement in the transverse direction results from the displacement of a metal ring arranged radially on the outside of the bladder are incompatible with the turning up of large lengths of carcass plies. All of the other known bladder devices in which the thrust is produced by another bladder or by the application of a laterally arranged mechanism cause, during their operation, a fanning of the carcass ply which is to be turned over. In fact, the carcass ply initially has a cylindrical configuration over its entire width, in particular between the bead wire and the side edge of the ply. Under the effect of the increase of pressure in the turn-up bladder, the edge of the carcass ply to be turned up, that is to say the part contained between the bead wire and the side end, lifts up gradually to pass through a stage where this carcass ply portion is arranged along a plane perpendicular to the axis of rotation. Thereupon, upon the rolling movement due to the thrust of the turn-up bladder, the carcass ply finally resumes a cylindrical shape. It will be noted that during the course of this movement the edge of the carcass ply suffers a very substantial elongation in circumferential direction, which elongation is greater the greater the transverse length of carcass ply to be turned up. This results in a stretching of the rubber calender-coating which is prejudicial to the quality of the tire.
The ply turn-up bladder in accordance with the teaching of U.S. Pat. No. 4,087,306 has, in addition, another drawback. It results from the fact that the turning-up movement due to the displacement of the bladder in the transverse direction commences at a very low inflation pressure of the bladder. When the inflation pressure is low, the force which the bladder is capable of developing on the carcass ply remains very low. In the case of metal carcass plies, it is insufficient to assure a uniform turning up at the base of the bead wire. When the turning up is effected, it is found that the carcass ply is not always properly applied against the products on which it is turned up, in particular in the vicinity of the bead wire where an empty space may remain.
All of the known turn-up bladders can be used only in order to turn up relatively short lengths of carcass plies. Now more and more frequently it is important to be able to effect ply turn ups of great length, sometimes up to 7 or 8 centimeters or even more.